1. Field of the Invention
The present invention relates generally to wireless communication systems and, in particular, to power of the forward link in wireless communication systems.
2. Description of the Related Art
Wireless communication systems employ Code Division Multiple Access (xe2x80x9cCDMAxe2x80x9d) modulation techniques to permit a large number of system users to communicate with one another. The ability of such a system to work is based on the fact that each signal is coded with spreading sequences, such as pseudo-random noise (xe2x80x9cPNxe2x80x9d) sequences, and orthogonal spreading sequences such as Walsh codes. This coding permits signal separation and signal reconstruction at the receiver. In typical CDMA systems, communication is achieved by using a different spreading sequence for each channel. This results in a plurality of transmitted signals sharing the same bandwidth. Particular transmitted signals are retrieved from the communication channel by despreading a signal from all of the signals by using a known user despreading sequence related to the spreading sequence implemented at the transmitter.
FIG. 1 illustrates CDMA system 10. The geographic area serviced by CDMA system 10 is divided into a plurality of spatially distinct areas called xe2x80x9ccells.xe2x80x9d Although cells 2, 4, 6 are illustrated as a hexagon in a honeycomb pattern, each cell is actually of an irregular shape that depends on the topography of the terrain surrounding the cell. Each cell 2, 4, 6 contains one base station 12, 14, and 16, respectively. Each base station 12, 14, and 16 includes equipment to communicate with Mobile Switching Center (xe2x80x9cMSCxe2x80x9d) 18, which is connected to local and/or long-distance transmission network 20, such as a public switch telephone network (PSTN). Each base station 12, 14, and 16 also includes radios and antennas that the base station uses to communicate with mobile terminals 22, 24.
When a call is set up in a CDMA system, a base station and mobile terminal communicate over a forward link and a reverse link. The forward link includes communication channels for transmitting signals from the base station to the mobile terminal, and the reverse link includes communication channels for transmitting signals from the mobile terminal to the base station. The base station transmits certain types of control information to the mobile terminal over a communication channel, referred to herein as a forward control channel, also known in the art as a forward overhead channel. Forward control channels include the pilot, paging, and synchronization channels, as well as other control channels. The base station transmits voice or data, and certain types of control information over a communication channel, referred to herein as a forward traffic channel. The signals on the communication channels are organized in time periods, referred to herein as frames. Frames are typically 20-millisecond (ms) in length. The signals transmitted over the control channels are referred to herein as control signals, and the signals transmitted over the traffic channels are referred to herein as traffic signals.
When a call is added to a cell, the noise level in the cell and in the surrounding cells is increased. If there is a large number of calls in a particular cell 4, it becomes difficult for mobile terminal 24 to clearly obtain the pilot and/or the forward-link traffic signal, particularly if mobile terminal 24 is at the edge of a cell. When mobile terminal 24 cannot obtain a clear and continuous pilot and/or the forward-link traffic signal, problems can result on the call between mobile terminal 24 and base station 14. These problems can range from not being able to despread a frame, which results in an erred frame, to the mobile terminal 24""s user hearing noise or silence instead of the voice or data that was transmitted, which results in an inconvenience to the user. If mobile terminal 24 cannot obtain a clear and continuous pilot and/or the forward-link traffic signal for a prolonged period or time, such as several seconds, the call may be dropped, which results in an inconvenience to the user and a loss of revenue.
When cell 4 is heavily loaded with calls, base station 14""s equipment may not be able to handle all of the calls in cell. This can occur when the power transmitted by the base station exceeds the power level at which the base station""s equipment is designed to operate over an extended time period. In some wireless communication systems 10, when there are many calls base station 14 initiates overload control. Base station 14 implements overload control by using one of several remedies. These remedies typically include: a) denying access to any new call requests, referred to herein as call blocking; b) restricting transmitted signals to their current levels; or c) even clipping transmitted signals. The inventors have discovered that this could occur even when other cells 2 and 6, may be able to accept new calls. This situation results in a loss of capacity of the overall wireless communication system 10.
The invention solves the above problems by adjusting the power level of a set of forward-link signals of a base station responsive to the loading of the forward link as determined by a power level measurement of the signal set. The power level of the signal set is adjusted independent of the individual power control of each of the forward-link signals in the set. Adjusting the power level of the signal set allows a cell that contains the base station to grow, i.e. cover a larger area, when the loading of the forward link is low. This allows a lightly loaded cell to accept calls from mobile terminals that may otherwise have been geographically constrained to a heavier loaded cell, thereby lightening the load in the heavier loaded cell. This also allows mobile terminals at the edge of cells to receive signals more clearly.
One power level measurement is a pilot fraction of the forward link, which is a ratio of the pilot""s power level to the power level of the forward-link signals. Other power level measurements, such as the signal set""s power level, can be used, alone or in combination, instead of or in addition to the pilot fraction of the forward link to adjust the power level of the signal set. Adjusting the power level of the signal set using several measurements involves determining how the power level of the signals set should be adjusted based on any of the power level measurements and adjusting the power level of the signal set when any one of the measurements indicates that the power level should be adjusted. Alternatively, the power level can be adjusted when several of the measurements indicate that the power level should be adjusted.
The power level of the set can be changed in any manner, including by scaling it by a scaling factor, or by increasing the power level by a fixed or a variable amount. The power level measurement of the signal set is obtained during a current time period. The scaling factor that will be used in the subsequent time period is determined using the power level measurement. In one embodiment of the invention, the scaling factor can be obtained from a look-up table that is based on the power level measurement.
If the cell containing the base station includes several sectors, the power level of the signal set in a sector is adjusted when the power level measurement in that sector indicates that the power level should be adjusted.